
NSF Org: |
EAR Division Of Earth Sciences |
Recipient: |
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Initial Amendment Date: | September 6, 2016 |
Latest Amendment Date: | September 6, 2016 |
Award Number: | 1644453 |
Award Instrument: | Standard Grant |
Program Manager: |
Robin Reichlin
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
Start Date: | May 15, 2016 |
End Date: | June 30, 2020 (Estimated) |
Total Intended Award Amount: | $95,653.00 |
Total Awarded Amount to Date: | $95,653.00 |
Funds Obligated to Date: |
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History of Investigator: |
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Recipient Sponsored Research Office: |
426 AUDITORIUM RD RM 2 EAST LANSING MI US 48824-2600 (517)355-5040 |
Sponsor Congressional District: |
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Primary Place of Performance: |
MI US 48824-2600 |
Primary Place of
Performance Congressional District: |
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Unique Entity Identifier (UEI): |
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Parent UEI: |
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NSF Program(s): | STUDIES OF THE EARTHS DEEP INT |
Primary Program Source: |
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Program Reference Code(s): | |
Program Element Code(s): |
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Award Agency Code: | 4900 |
Fund Agency Code: | 4900 |
Assistance Listing Number(s): | 47.050 |
ABSTRACT
The lowermost part of the earth's mantle, referred to as the D" region, is a dynamic region that is both a thermal and chemical boundary layer between the solid, silicate mantle and the fluid, mostly iron outer core. A better understanding of the deformation processes that occur in this region would provide important constraints on the current dynamics of the entire mantle, the processes of heat transfer from the core to the mantle, the thermal evolution of our planet, and the existence and extent of geochemical heterogeneity. To study deformation processes in the deepest mantle, the investigators combine expertise from several disciplines: seismology, mineral physics and geodynamical modeling of mantle convection, linked together around a common object of study: seismic anisotropy, i.e. the difference in propagation speeds of seismic waves depending on the orientation of the path travelled, a proxy for macroscopic deformation. The latter's characteristics reflect mineral properties as well as flow strength and geometry. In this project, the team will apply the multi-disciplinary tools developed in a previous joint study funded by the CSEDI program of NSF to further characterize the possible causes of seismic anisotropy in the earth's deep mantle. They will begin with 3D fluid dynamical modeling of mantle convection, in which they will model the deformation of descending tectonic plates as they come in contact with Earth's core-mantle boundary. It is unclear how strong these plates are, and their strength will control how they deform. Strong plates will buckle and bend, whereas weak plates will deform is a more ductile fashion. The team will examine numerous scenarios, each assuming a different strength for descending plates. They will also examine scenarios in which descending plates interact with hypothesized compositional heterogeneity in the deep mantle. Deformation data from the dynamical calculations will be used as input for mineral physics calculations to predict the alignment of minerals which will control the nature of seismic anisotropy. By comparing predicted seismic anisotropy from various models to that observed by seismic studies, they will constrain deformation characteristics of descending plates and compositional characteristics of the D" zone. This will provide important information on how sinking plates drive larger-scale mantle convection.
The presence of anisotropy in the D" region of the earth's mantle is now well established, although its cause remains unclear. Much progress was recently achieved in mineral physics, to characterize elastic and deformation properties of lowermost mantle minerals including the post-perovskite (pPv) phase, as well as in geodynamics, tracking strain evolution in mantle convection modeling. There are now precise ways to compute synthetic seismograms in a 3D anisotropic earth down to body wave frequencies. This study will advance our understanding of the structure and dynamics of an important boundary layer region in the earth. In previous collaborative work funded by CSEDI, the investigators developed a multi-disciplinary approach combining elements from geodynamic modeling, mineral physics and material science experiments and computations, to perform forward modeling of crystal preferred orientation (CPO) anisotropy in a 3D spherical earth, in the deep mantle part of a subducted slab, under different starting assumptions, and compared them with seismic observations. The ultimate goal is to gain better understanding of the origin of seismic anisotropy in D", and determine which microscopic and macroscopic processes may or may not be at play. So far, they investigated the case of a 3D geodynamical model under rather simple rheological assumptions. Now, they will explore varying rheologies producing slabs of variable strength, including the effect of the pPv phase-change, and how slabs will deform in the presence of hypothetical thermochemical piles. For each of these different calculations, they will provide the deformation information to serve as input for the mineralogical texture development within a polycrystalline mineral aggregate. While the team will still focus on three phases, perovskite, pPv and ferropericlase, they will also explore the effect of a variety of Fe and Al substitution mechanisms, both theoretically and experimentally. The predicted seismic anisotropy from these models will be confronted with seismological observations of radial and azimuthal anisotropy both acquired during this project and from the literature.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
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This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
It is generally recognized that plate tectonics on Earth’s surface is caused by very slow convection of the interior mantle beneath the plates. While we now have a good kinematic understanding of how fast and in what directions the plates are moving on the surface through techniques such as GPS, we do not have a good understanding how the convection works, particularly in the deeper regions of Earth’s mantle. We can image the interior using techniques such as seismic tomography to get a snapshot image of present-day locations of hotter and cooler portions of the mantle, but this is a blurry picture at best. Also, while seismic tomography can show us what the interior looks like at this moment in time, it doesn’t provide any information on the speeds or directions of mantle flow. There are other observations that do however, such as seismic anisotropy.
The Earth’s mantle is made of minerals that are seismically anisotropic, meaning seismic waves can move through them faster in one direction than the other. Typically, one would expect that mineral grains are randomly oriented as the mantle is convecting, therefore, canceling out this anisotropy as a seismic wave travels through many minerals. However, under some conditions, such as high stress, or if the mineral grains exceed a certain size, the microscopic details of how the mantle convects changes. At lower stresses, the minerals remain randomly oriented as the mantle flows, but at higher stresses (or larger grain sizes), the mantle flow itself starts to preferentially align the mineral grains. Once there is statistical alignment of mineral grains, we observe the seismic anisotropy on a large scale.
Most of the lower mantle does not exhibit seismic anisotropy, except for at the bottom, right above the liquid iron core. Furthermore, we typically see this anisotropy in the areas beneath subduction zones, places where tectonic plates sink back into the Earth. Therefore, it is hypothesized that the relatively cool tectonic plates that sink into the mantle undergo a higher stress, causing mineral alignment. Most of the alignment is expected to occur at the bottom of the mantle because of the higher amount of strain that occurs as the plate impacts the core.
The larger goal of this work is to better understand the connections that link deformation of sunken plates in the lower mantle to the seismic anisotropy observations we see. This basically involves three components: (a) understanding how the sinking plates deform at the macro-scale (10s to 100s of kilometers), (b) understanding how that macro-scale strain influences minerals at the micro-scale (millimeters) to align them, and (c) how seismic waves travel through the mineral fabrics created at the micro-scale to generate seismic anisotropy observed at the macro-scale. By integrating these processes, we can explore how different deformational styles of subducted plates can lead to different observations of seismic anisotropy, which will better help us understand how to interpret these observations.
This particular portion of the project involves using computational fluid dynamics models to simulate the sinking of subducting plates into Earth’s mantle. We investigate 2 controlling factors: how wide the plate is and how strong the plate is. We can measure how wide the plate is at the surface; however, we don’t have a good understanding of how strong the plate it, particularly after it reaches the lower mantle. Through fluid dynamical modeling, we examine how the strength of the plate in the lower mantle controls the mantle convection flow patterns at the bottom of the mantle. We find that weaker plates produce flow patterns that spread out in a relatively azimuthal fashion above the core, whereas stronger plates produce flow patterns that are unidirectional or bi-directional. Each of these different flow patterns would lead to different observations of seismic anisotropy. This allows us to ultimately link observations of seismic anisotropy to the strength of a plate in the lower mantle. By better understanding the strength of plates in the lower mantle, we better understand the material properties of the lower mantle, which in turns provide us with a better understanding of how mantle convection works at the large-scale and causes plate tectonics.
Last Modified: 12/31/2020
Modified by: Allen Mcnamara
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